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Full-Duplex Digital Communication on a Single Laser Beam

The laser beam would be transmitted with one modulation and retroreflected with another modulation.

Goddard Space Flight Center, Greenbelt, Maryland

A proposed free-space optical communication system would operate in a full-duplex mode, using a single constant-power laser beam for transmission and reception of binary signals at both ends of the free-space optical path. The system was conceived for two-way data communication between a ground station and a spacecraft in a low orbit around the Earth. It has been estimated that in this application, a data rate of 10 kb/s could be achieved at a ground-station-to-spacecraft distance of 320 km, using a laser power of only 100 mW. The basic system concept is also applicable to terrestrial free-space optical communications.

The Laser at One End of the free-space optical path would provide all of the beam power needed for transmission of data signals in both directions along the path.
The system (see figure) would include a diode laser at one end of the link (originally, the ground station) and a liquid-crystal- based retroreflecting modulator at the other end of the link (originally, the spacecraft). At the laser end, the beam to be transmitted would be made to pass through a quarter-wave plate, which would convert its linear polarization to right circular polarization. For transmission of data from the laser end to the retroreflector end, the laser beam would be modulated with subcarrier phase-shift keying (SC-PSK). The transmitted beam would then pass through an aperture- sharing element (ASE) — basically, a mirror with a hole in it, used to separate the paths of the transmitted and received light beams. The transmitted beam would continue outward through a telescope (which, in the original application, would be equipped with a spacecraft-tracking system) that would launch the transmitted beam along the free-space optical path to the retroreflector end.

At the retroreflector end, a portion of the received laser beam would be sent to a demodulator for detection of the SC-PSK signal. For transmitting data to the laser end, the retroreflected portion of the received laser beam would be modulated with circular-polarization keying (CPK), in which left circular polarization signifies a binary level (“1” in this case) and right circular polarization signifies the other binary level (“0” in this case). Hence, to transmit “0,” the retroreflecting modulator would leave the right circular polarization of the retroreflected beam unchanged; to transmit “1,” the retroreflecting modulator would flip the polarization of the reflected beam to left circular. Full-duplex operation would be possible because the CPK and the SC-PSK would be transparent to each other.

At the laser end, the reflected, CPK-modulated beam would return through the telescope and would then be reflected by the ASE into a receiver subsystem. A beam splitter would divert 0.2 percent of the beam power to a camera in the tracking system. The remainder of the beam would pass through the beam splitter to a quarter-wave plate, which would convert the circular polarization to two orthogonal linear polarizations. A polarizing beam splitter would then split the light in these two polarizations so that photons corresponding to “0” would go to one photodetector and photons corresponding to “1” would go to another photodetector.

It should be emphasized that this arrangement would yield a nonzero photodetector output of nominally the same magnitude for either “0” or “1.” This is fundamentally different from on-off keying (OOK), in which “0” or “1” is represented by the absence or presence, respectively, of a signal. Taking advantage of this, prior to final digitization of the return signal at “0” or “1,” the output of the “0” photodetector could be inverted, then subtracted from the output of the “1” photodetector to obtain twice the signal-to-noise ratio achievable in OOK.

The receiver subsystem would include Faraday-anomalous-dispersion optical filters (FADOFs), which would reject background light to such a high degree that the system could operate over a long path during daytime. The FADOFs would essentially prevent skylight from reaching the photodetectors while allowing about 80 percent of the signal photons to pass through. Without the FADOFs, it would be necessary to increase the laser power by a factor of 10 for daytime operation.

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